1. Field of the Invention
The present invention relates to a process for fabricating a semiconductor device. More specifically, the present invention relates to an electrical or ohmic contact of an aluminum or aluminum-alloy wiring layer with a semiconductor substrate such as silicon, using a refractory metal silicide film as a barrier film between the wiring layer and the substrate.
2. Description of the Related Art
Aluminum or an aluminum-alloy such as aluminum-silicon is widely used as a wiring material in integrated circuits. These materials have the characteristics of low resistivity and good adherence to silicon or silicon oxide and are capable of forming ohmic contact with p-type and n-type doped layers. It is well known, however, that when heat treatment is carried out during the semiconductor device fabrication process, aluminum is dissolved into the silicon substrate, forming vertical aluminum-rich spikes passing through a pn-junction in the silicon substrate, and causing junction leakage or other defects. This problem becomes more serious when a dopant-diffused region is made shallower and narrower in order to increase the density of the integrated circuits.
To alleviate the above-described problem, aluminum-silicon alloy, typically 1%-silicon aluminum-alloy, is used as the wiring material in integrated circuits. However, when heat treatment is carried out during the fabrication process, the silicon in an aluminum-silicon alloy wiring layer is deposited onto the surface of a silicon substrate by solid phase epitaxy. This deposition may cause contact failure or breaks in the wiring. This silicon deposition has a p-type conductivity due to the aluminum content and, therefore, contact failure occurs easily when the substrate has an n-type conductivity. Further, the problem caused by this deposition becomes more serious when the size of a contact window between the wiring and the substrate is made smaller, in particular, smaller than a square measuring 2 .mu.m.times.2 .mu.m, in order to increase the density of the integrated circuits.
A barrier film can be inserted between an aluminum or aluminum-alloy wiring layer and a silicon semiconductor substrate in a contact region, to avoid the above-described problems of the aluminum and aluminum-silicon-alloy wiring layers, and silicides of refractory metals, typically, molybdenum silicide (MoSi.sub.2), are proposed to be used for such a barrier film.
However, in known methods for the deposition of silicide, such as co-sputtering, hotpress-target-sputtering, and the like, the refractory metal silicide layer as deposited is not well-formed. In experiments, the inventors found that, if an aluminum or aluminum-alloy wiring layer is formed onto a film made by depositing both a refractory metal such as molybdenum and silicon on a semiconductor substrate, without carrying out annealing of the film before forming the wiring layer, a silicon phase may appear in the aluminum or aluminum-alloy wiring layer formed on the barrier film when heat treatment is carried out during the fabrication process. If this silicon phase is formed in an aluminum or aluminum-alloy wiring layer, electromigration is accelerated in the wiring layer, resulting in wiring defects or breakage.
There has been proposed the annealing of a barrier film made by depositing both a refractory metal and silicon on a semiconductor substrate, the annealing being carried out in a furnace before forming an aluminum or aluminum-alloy wiring layer on the film, in order to fix the bonds between the refractory metal and the silicon, or to silicidate the refractory metal. For example, this furnace annealing for silicidating a barrier metal film comprising molybdenum and silicon is carried out at a temperature of 900.degree. C. for 20 minutes in a nitrogen atmosphere in a diffusion furnace. It is known that, ideally, a temperature of 1050.degree. C. is needed to ensure silicidation of molybdenum. However, if such a furnace annealing is carried out at a temperature of 900.degree. C. to 1050.degree. C., an excellent electrical or ohmic contact between the wiring layer and the semiconductor substrate is difficult to obtain, or is not stably obtained, resulting in defects occurring in the semiconductor devices.